In a typical powder coating apparatus, powder is maintained in a fluidized state within a hopper having a fluidized bed, and is transported from the hopper to a pump chamber of a powder pump, whereupon an ejector nozzle in the pump directs a high pressure gas stream toward an outlet of the chamber and along an exit tube connected to the chamber. As a result, the powder is conveyed through the exit tube to a spray gun which sprays the powder toward the surface of an article to be coated. The ejector nozzle must be operated at sufficiently high pressure to transport the particles to the end of the exit tube and through the spray gun. During this pumping operation, the high pressure oft he ejector nozzle creates a relatively low pressure at the intake to the pump chamber, thereby drawing powder particles from the powder container and into the chamber for ejection therefrom along the exit tube.
One typical application for an apparatus of this type involves coating the surface of an aluminum joint with powdered solder flux prior to steps of heating and melting the solder flux to weld the connecting aluminum elements and form the joint. The reliability of the connection depends upon the uniformity of the flux coating.
In these and other applications wherein powder particles are gas-transported to coat a surface, including applications which involve electrostatic charging of the particles, problems may result due to adherence and accumulation of the powder particles at the inlet of the pump from the hopper. This occurs with powders that have the property of relatively easy coherence, easy adherence due to viscosity, relatively low slipperiness, or powders which simply do not have good flow characteristics and tend to agglomerate. Powdered solder flux is such a powder. With these types of powders, even powder flow from the hopper to the spray gun is difficult to achieve, and therefore, the amount of powder transported per unit time fluctuates. As a result, it is difficult when spraying these powders to achieve a uniformly thick coating of powder on the article and to consistently apply a uniform coating from one article to the next in a production line situation. This is possibly due to changes in the flow of the powder resulting from a reduction in the cross sectional area at the inlet to the pump or because of the influence of static electricity among the particles due to friction which encourages agglomeration of the powder particles.
It is therefore one objective of the invention to achieve greater uniformity in powder transport per unit time by minimizing or reducing accumulation and adherence of the particles during transport from the powder hopper to the article to be coated.
In other applications for powder coating via gas transport, it is often necessary to intermittently turn the powder pump on and off. One example for the need of this type of operation involves coating articles carried on a conveyor, wherein it is desirable to spray coat the articles at a coating station on the conveyor, and then turn the pump off until the conveyor moves the next article to the coating station. For applications which require ON/OFF operation of a powder coating apparatus, it is desirable to achieve precise control of the powder flow to effectively turn the apparatus off and on at the desired times. Otherwise, powder is wasted. It is also desirable to spray the same quantity of powder on each article.
In the past, pinch valves made of rubber tubes have been used to control intermittent powder flow at a powder hopper. Rubberized pinch valves are pneumatically operated and are relatively simple and inexpensive. However, during closing, these valves have a tendency to close upon some powder particles. Eventually this causes gaps between the opposing rubber portions and produces air leaks. These leaks allow some powder to move to the downstream side of the pinch valve. Because there is no way of knowing how much powder has reached the downstream side of the pinch valve, the amount of powder ejected during each ON/OFF cycle may vary. This will produce nonuniformity in coating an article. Additionally, after a certain number of switching operations, the rubber of the pinch valve becomes fatigued and it deteriorates to a point where it is impossible to use. Again, as this occurs, the effectiveness of the valve becomes questionable. As varying quantities of powder reach the downstream side, the apparatus will eject varying quantities of powder during each ON/OFF cycle.
It is another objective of this invention to more precisely control the ON/OFF switching operation of a powder pump, thereby to assure delivery of uniform powder quantities during each ON/OFF cycle of operation.
It is still another objective of the invention to simultaneously achieve uniform powder ejection per unit time during an ON cycle and to effectively stop and start powder ejection during switching operation between ON/OFF and OFF/ON, respectively.
It is still another objective of the invention to minimize the adverse effects of powder cohesion, adherence, agglomeration and friction during flow from a fluidized powder hopper to an outlet end of a spray gun, thereby to achieve improved uniformity in powder delivery to an article to be coated and to produce a more stable coating thereon.